It is desirable to operate distillative processes at minimum energy, to effect separation of the feed stream into a desired overhead product stream and a bottom product stream in a distillative column having vapor-liquid contacting devices, such as distillation trays, packing devices or a combination thereof. In typical distillative processes, the overhead product stream is at least partially condensed and a small portion recycled to the top of the distillative column, while the bottom product stream is withdrawn and reboiled and at least a portion recycled to the bottom of the column, to provide desirable column operating conditions. Some distillative columns operate under such conditions, so as to obtain the desired overhead product stream of defined specifications enriched in a desirable component, or conversely to obtain a bottom product stream of defined sepcifications enriched in a particular component, or both, so as to obtain purified streams for further separation or recovery or use in a chemical, refinery or petro-chemical operation. In any event, such distillative techniques should be carried out at the most desirable column operating conditions, wherein optimum energy savings can be effected.
It is known that the separation of a feed stream in a distillative column, particularly a gaseous hydrocarbon feed stream comprising methane and an acid gas component, such as carbon dioxide, may be separated efficiently through the use of a nonpolar additive agent, such as a liquid additive agent; for example, a C.sub.3 -C.sub.6 alkane, particularly butane-plus, introduced into the upper portion of said distillative column in an amount to prevent the formation of solids in the cryogenic distillation of methane from carbon dioxide, such as, for example, as more particularly set forth in U.S. Pat. No. 4,318,723, issued Mar. 9, 1982 (hereby incorporated by reference).
Also, it has been known that, in the prevention of azeotropic formation between ethane and carbon dioxide and ethylene and hydrogen sulfide and other components, the introduction of a nonpolar additive agent, such as a liquid hydrocarbon additive agent, such as a C.sub.3 -C.sub.6 alkane, particularly butane-plus, prevents or inhibits the formation of azeotropes and enables the separation to provide an overhead product stream more enriched in a desirable component and a bottom product stream more enriched in a bottom product component, through the alteration of the azeotropic formation, such as, for example, as set forth and described more particularly in U.S. Pat. No. 4,350,511, issued Sept. 21, 1982 (hereby incorporated by reference).
Also, it is known to change the relative volatility of acid gas components, such as carbon dioxide to hydrogen sulfide, through the use of an additive agent, such as a nonpolar liquid additive agent, such as hydrocarbon; for example, a C.sub.3 -C.sub.6 alkane, particularly butane-plus, in order to enhance the relative volatility of the carbon dioxide and hydrogen sulfide and, therefore, to increase the efficiency of separation, as set forth in U.S. Pat. No. 4,293,322, issued Oct. 6, 1981, now U.S. Pat. No. 4,383,842, issued May 17, 1983.
U.S. patent application Ser. No. 307,672, filed Oct. 1, 1981 (hereby incorporated by reference) relates to an improvement in the effective separation of methane from carbon dioxide in a distillative column, wherein the upper portion of the column is operated at temperatures above the triple point of carbon dioxide; that is, -70.degree. F., by increasing the amount of nonpolar additive agent added to the reflux condenser, to maintain the reflux condenser and all portions of the column above the triplepoint temperature.
In all of the prior operations, the resulting bottom product stream contains, in addition to the usual bottom product stream components, a liquid nonpolar additive, particularly the liquid hydrocarbon additive added to change the operating conditions in the column. The liquid additive agent may be recycled with the bottom product stream or may be separated and recycled for use in any one or all of the aforementioned uses of an additive agent, particularly where such distillative processes are employed in one-, two-, three- or multiple-column operations for the separation of a natural gas stream or petro-chemical stream into the desired components. The feed streams employed in such distillation process include those streams which have major amounts of an acid gas component desired to be removed and those streams containing minor amounts of or even no acid gas components. It would be desirable, in such distillative separations or a combination of operations and other distillative operations where additives are not used, to reduce the energy requirements of such distillation techniques.